Sensing technology is currently employed in a number of different environments. Specifically, sensors employed to determine the pressure or temperature of a medium are used in a wide variety of applications. Most such applications involve the use of temperature and pressure sensors in environments of low temperature or in environments of high temperature which require adequate cooling measures or the use of high temperature materials in the construction of such sensors. In such applications, for example, micro-machining techniques exist by which pressure sensors are constructed using silicon as a substrate.
An example of a micromachined silicon pressure sensor design is a capacitive pressure sensor. This sensor uses a parallel plate capacitor and a flexible silicon diaphragm. Two silicon wafers are bulk machined to create cavities in the silicon. One silicon wafer is bulk micromachined to create a deep cavity and subsequently a thin membrane. Metal layers are deposited onto appropriate boundaries of the cavities creating the conductors of the parallel plate capacitor. The wafers are bonded so that the metal conductors are facing each other and a capacitor is formed. The capacitor is electrically connected to a silicon circuit on the substrate which in turn is connected to external electronic devices via wire leads. As pressure of the medium in which the sensor is placed increases, the diaphragm deflects and the distance between the plates of the capacitor decreases, causing an increase in the capacitance. The change in capacitance is read by the silicon circuit, and a resultant voltage is output via the wire leads.
Micromachined sensors such as the example given above suffer problems when exposed to certain environmental conditions. In high temperature applications, the silicon sensor and similar sensors do not operate reliably or cease to function completely due to the heat. For example, silicon begins to plastically deform at approximately 800.degree. C. and melts at approximately 1400.degree. C. The pressure readout due to the deflection of the flexible silicon diaphragm is compromised by the plastic deformation of silicon causing permanent measurement error. Many other sensor materials have even lower melting points which will limit the operating temperature of the environment. In addition, different environments may include corrosive elements in which silicon or other similar materials may not survive.
Another problem with micromachined silicon sensors and similar sensor technology is that circuitry, electrical connections, and wire leads through which temperature, pressure, or other physical information is obtained can not withstand high temperature applications or corrosive environments. For example, silicon circuitry does not function at temperatures greater than 300.degree. C. and high temperature solders, conductive adhesives, and wiring schemes are difficult to implement.
In addition, in the case where a temperature, pressure, or other physical reading of an environment is measured from a sensor mounted to a mobile structure such as a turbine blade or other moveable apparatus, chamber or vessel, the wire leads connected to traditional sensors may interfere with the operation of the particular mobile structure. Such would also be the case of mobile vessels in which interior pressure sensing is desired.
Consequently, there is a need for pressure and temperature sensing technology which overcomes the problems experienced by traditional sensors as described above.